1. Field of Invention
The present invention relates to a motor unit applicable to electrical equipment for motor vehicles or the like.
2. Description of Related Art
In general, motor units of this kind include a motor equipped with a rotation detecting mechanism for detecting the quantity of rotation (number of revolutions) of an output shaft of a motor to detect the quantity of movement or the position of a movable member operating in accordance with the drive of the motor, one known prior example of which is shown in FIG. 8.
The motor unit exemplified in FIG. 8 is designed to detect the rotation of a motor output shaft 11 through the use of a sensor magnet 18 whose N and S poles are alternately polarized along its circumference and a reed switch 20 that switches between ON and OFF in response to the variation of the magnetic field caused by the rotation of the sensor magnet 18. In this motor unit, a magnet holder 22 fixedly supporting the sensor magnet 18 is fitted over the output shaft 11 in a locked condition to be rotatable integrally with the output shaft 11. The magnet holder is further rotatably supported on a case cover 19 attached to the opening side of a gear case 9 that rotatably supports the output shaft 11. The reed switch 20 is installed on the case cover 19 and separated by a predetermined distance from the sensor magnet 18, and the contact of the reed switch 20 switches between ON and OFF in accordance with the variation of the magnetic field due to the rotation of the sensor magnet 18 so that this switching signal is converted into a pulse signal to be counted, thereby allowing the detection of the quantity of rotation (number of revolutions) of the output shaft 11.
In the foregoing conventional motor unit, the magnet holder fixedly supports the sensor magnet and is fitted over the output shaft in a locked condition as mentioned above, and further is pivotally supported on a bearing section of the case cover. Accordingly, because of manufacturing and assembling errors, for instance, if the magnet holder is installed in the case cover bearing section in an inclined condition, a large load can be generated at a rotational sliding portion between the case cover bearing section and the magnet holder, which can cause deformation of the sliding portion or premature wear of the sliding portion.
In addition, if the magnet holder is in the inclined condition as mentioned above, the distance between the sensor magnet and the reed switch varies in accordance with the rotational position of the sensor magnet, thus resulting in production of unstable pulses.
In the foregoing related art, the magnet holder 22 is positioned in an axial direction between a lock ring 14 fixedly attached to the output shaft and the case cover 19. The magnet holder can be shifted in position in its axial direction or become loose due to the dimensional errors between the respective parts, or the like. In such an instance, the relative position between the sensor magnet and the reed switch can be shifted, which also makes it difficult to obtain stable pulses.
As disclosed in Japanese Unexamined Utility Model Publication No. 7-27272, a drive member (corresponding to the aforesaid magnet holder) connected to an output shaft and a sensor magnet are interrelated with a given separation therebetween so that the separation can absorb the above-mentioned inclination. However, this construction can create problems in that the sensor magnet can become loose in circumferential directions by an amount corresponding to the separation with respect to the output shaft and the reed switch. Furthermore, the positional shifting in the circumferential directions occurs between the output shaft and the sensor magnet, which causes errors in rotation detection. This and other problems are solved by the Applicant's invention.
In the structure disclosed in the aforementioned publication, the regulation of the position of the magnetic holder in its axial directions is made through the use of a special fixing ring and wavy washer. However, in this case, there is a problem that the number of parts and the number of assembling steps increase.